Recently, as people become more concerned about environmental problems, global warming and the depletion of oil resources have become major issues.
With that as background, there is a need for naturally recycling environmentally friendly materials composed of biomass, which decompose into water and carbon dioxide after use. Polylactic acid has attracted the most attention among biodegradable polymers using biomass. Polylactic acid is a kind of aliphatic polyester using, as raw material, lactic acid obtained from the fermentation of starch extracted from plants and it presents the best balance of dynamical characteristics, heat resistance and cost efficiency of biodegradable polymers using biomass. Therefore, resin products, fibers, films, sheets and other such materials utilizing it are being rapidly developed.
The application development of polylactic acid fibers has already commenced with agricultural materials, civil engineering materials and so on, which profit from biodegradability; however its application in apparel, curtains, carpets and other interior goods, or car interior goods, and other such industrial materials is also expected to be large application fields.
Howsoever, the polylactic acid fibers did have inconveniences such as a high surface friction coefficient and low wearing resistance. For instance, wearing resistance in accordance to JIS L 0849 is generally required to be of grade 3 or better, but that of conventional polylactic acid fibers was as low as grade 1.
Consequently, the development of applications requiring wearing resistance such as apparel, interior goods, car interiors and the like has not progressed very far. For instance, if conventional polylactic acid fiber are used in applications such as outerwear, uniforms, sport wear, the quality degrades through fluffing, whitening, shine and so on in the shoulders, elbows, knees, hips or the like which are often submitted to wearing in daily life, or color migration to innerwear and other such problems. In addition, also in the application to upholstery, carpets and so on, durability was low, causing fluffing, wear and tear of fibers through repeated wearing and color migration to clothing that includes trousers, socks and so on, and other such problems have been encountered.
The high surface friction coefficient of polylactic acid fibers has also caused problems in the yarn-making, yarn treatment, cloth cutting and sawing processes.
In the melting and spinning process, fluffing or yarn being easily cut occur because of large friction quotient between the yarn and the guide or the like, when the yarn travels at the high speeds of 1000 to 7000 m/min. And on the other hand, the yarn twines around the roller and easily breaks, in the drawing process. Yarn break and fluffing often occur due to friction between the yarns and the twisted body, or between yarns, during the yarn treatment process, particularly the temporary standing process, largely decreasing ability to smoothly pass through processing steps and the quality of the product.
In general, in the industrial fabric cutting process a plurality of clothing items are superposed and then cut and here in the case of cloth using conventional polylactic acid fibers, they tend to fuse with each other because of the large shearing heat generated between the cutter and the fiber.
In the sawing process, at normal sawing speed, fibers fuse thus deteriorating the quality of the product, through the generation of friction heat between the machine needles and the fibers, or polymer deposits on the machine needles cause the necessity for frequent replacement of machine needles, therefore, lowering processing speed, and subsequently the productivity.
It is considered that the high friction coefficient of polylactic acid fibers causing these problems depends on the polymer matrix, and that these problems are inevitable with polylactic acid fibers.
Incidentally, in the field of resin products, films, sheets and the like, smoothing agent is sometimes added to the polymer in the manufacturing process, in order to improve the anti-blocking characteristic of chips or fused polymers or to facilitate the release of the mold from the die or roller. However, in the field of fibers, the addition of smoothing agent has been avoided, because fiber macula, dying macula or other product quality deteriorations easily occur from blending unevenness, heat decomposition, bleed out or the like of the smoothing agent.
Examples of smoothing agent addition to fiber are extremely few; however, for example, Japanese Patent Laid-Open Publication No. 1996-183898 discloses one. The technology concerned consists of adding a fatty monoamide, expressed by the general formula RCONH2 (here, R represents an alkyl group), to polylactic acid fibers and has the objective of suppressing hydrolysis speed by imparting water repellency. However, there is no description concerning the improvement of wearing resistance and ability to smoothly pass through processing steps of polylactic acid fibers, which is the objective of the present invention. To be sure, the Inventors did double-check polylactic acid fibers in which a fatty monoamide had been added, however, wearing resistance and ability to smoothly pass through processing steps of polylactic acid fibers could not be improved upon (Cf. Comparative examples 4, 5). The Inventors inferred that the cause thereof is the fact that the fatty monoamide reacts with polylactic acid during fusion, because the amide group thereof is highly reactive and as a result, the proportion of fatty monoamide that can function as smoothing agent in fibers of fatty monoamide decreases.
In some cases, when a fatty monoamide reacts with polylactic acid, molecular chains of polylactic acid end up cut and consequently molecular weight decreases, lowering the fiber properties.
Moreover, fatty monoamides, having high sublimation property or low heat resistance, sometimes cause deterioration of the work environment through smoke, fouling of guides, rollers or the like through bleeding out, and deterioration of operating efficiency. Furthermore, bled out fatty monoamide condenses on the fiber surface, sometimes causing fiber property macula or dying macula.
Consequently, the present invention has the objective of providing polylactic acid fibers that excellent wearing resistance and ability to smoothly pass through processing steps.